Made a false assumption based on Xbitlabs' misleading understanding of PCWatch's article.
The specific device that handles 25A and 16 phases per cell is a research chip. No known details for Haswell's FiVR exist.
I think what "Fully Integrated" refers is that the single die is capable of handling multiple voltage rails.
As for the version shown in Xbitlabs: They say that only 3 cells active are needed to fully power a 90W Xeon without problems. 20 cells indicate 500A, and if they integrate a derivative of that, there's no way you'd run out of power headroom because of the current "limitations". Also, having much higher phases would allow stability, potentially increasing overclocking potential.
320 phases is 20x what high end motherboards have.
Fixed typo.